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Abstract:

Use of an acoustic wave receiving apparatus which includes: a resonator
including a first mirror on which measurement light is incident, a second
mirror which is arranged to face the first mirror and on which acoustic
waves from an object are incident, an acoustic wave reception layer
interposed between the first mirror and the second mirror, and a
compensation layer; and a detector for detecting a variation in an
optical path length between the first mirror and the second mirror that
occurs in response to deformation of the acoustic wave reception layer
caused by incidence of the acoustic waves, wherein the variation in the
optical path length due to a film thickness distribution of the acoustic
wave reception layer is compensated by refraction in the compensation
layer.

Claims:

1. An acoustic wave receiving apparatus comprising: a resonator including
a first mirror on which measurement light is incident, a second minor
which is arranged to face said first minor and on which acoustic waves
from an object are incident, an acoustic wave reception layer interposed
between said first minor and said second mirror, and a compensation
layer; and a detector for detecting a variation in an optical path length
between said first minor and said second mirror that occurs in response
to deformation of said acoustic wave reception layer caused by incidence
of the acoustic waves, wherein the variation in the optical path length
due to a film thickness distribution of said acoustic wave reception
layer is compensated for by refraction in said compensation layer.

2. The acoustic wave receiving apparatus according to claim 1, further
comprising a controller for controlling said compensation layer, wherein
said controller compensates for the variation in optical path length due
to film thickness distribution of said acoustic wave reception layer, by
varying the refractive index of said compensation layer.

3. The acoustic wave receiving apparatus according to claim 2, wherein
said detector detects a variation in the optical path length by using
wavelength of the measurement light, film thicknesses and refractive
indices of said acoustic wave reception layer and said compensation
layer, and variation in a reflected light amount of the measurement
light, and said controller compensates the variation in the optical path
length by varying the refractive index of said compensation layer
according to a film thickness distribution of said acoustic wave
reception layer.

4. The acoustic wave receiving apparatus according to claim 2, wherein
said compensation layer is layered with said acoustic wave reception
layer.

5. The acoustic wave receiving apparatus according claim 2, further
comprising a signal processor for obtaining an intensity of acoustic
waves from the object based on the variation in the optical path length
detected by said detector.

6. The acoustic wave receiving apparatus according to claim 5, wherein
said controller divides said acoustic wave reception layer into a
plurality of regions according to the film thickness distribution thereof
and determines a refractive index of said compensation layer for each of
said regions such that the optical path length is substantially constant
in the same region.

7. The acoustic wave receiving apparatus according to claim 6, wherein
said controller sequentially controls each of the plurality of regions
such that the refractive index of said compensation layer is equal to
each of the plurality of refractive indices determined for the respective
regions, said detector performs measurement at each of the refractive
indices controlled by said controller, and when obtaining an intensity of
acoustic waves from the object, said signal processor uses, as the
variation in the optical path length in each of the plurality of regions,
the value measured by said detector when the refractive index of said
compensation layer is a refractive index corresponding to that region.

8. The acoustic wave receiving apparatus according to claim 6, wherein
said controller compensates for the variation in the optical path length
by performing control such that the refractive index of said compensation
layer differs for each of plural regions corresponding to said plurality
of regions of the acoustic wave reception layer.

9. The acoustic wave receiving apparatus according to claim 8, wherein
said compensation layer is a simple-matrix driven or active-matrix driven
liquid crystal, and said controller controls the voltage applied to
pixels of said compensation layer.

10. The acoustic wave receiving apparatus according to claim 1, wherein
said compensation layer has a refractive index distribution according to
the film thickness distribution of said acoustic wave reception layer,
such that the variation in the optical path length due to the film
thickness distribution is compensated for.

11. The acoustic wave receiving apparatus according to claim 10, wherein
said compensation layer is made of a liquid crystal material in which an
orientation state of liquid crystal molecules is fixed.

12. The acoustic wave receiving apparatus according to claim 10, wherein
said compensation layer is made of an organic substance or charged
material which has different refractive indices according to
concentration gradients.

13. The acoustic wave receiving apparatus according claim 1, wherein the
acoustic waves from the object are photoacoustic waves generated when
excitation light is emitted to the object.

14. The acoustic wave receiving apparatus according to claim 13, wherein
the excitation light serving as a trigger is emitted to the object at a
predetermined frequency, and said detector performs measurement of the
photoacoustic waves by emitting the measurement light to said detector
while delaying the emission each time by a predetermined timing from the
trigger in a cycle in which the excitation light is emitted.

Description:

TECHNICAL FIELD

[0001] This invention relates to an acoustic wave receiving apparatus.

BACKGROUND ART

[0002] In general, imaging apparatuses using X-rays, ultrasound waves, and
magnetic resonance imaging (MRI) are widely employed in the field of
medicine. On the other hand, in the field of medicine, researches have
been actively carried out to develop apparatuses employing an optical
imaging technology in which light from a light source such as a laser is
emitted to and propagated in a test object such as a living body, so that
information in the living body is acquired by detecting the propagated
light.

[0003] Photoacoustic Tomography (PAT) has been proposed as one of such
optical imaging technologies. In PAT, pulsed light generated by a light
source is emitted to a test object, whereby acoustic waves (hereafter,
also referred to as photoacoustic waves) are generated by living body
tissues which have absorbed optical energy propagated and diffused inside
the test object. These photoacoustic waves are detected at a plurality of
positions, and photoacoustic signals thus obtained are analyzed and
processed so that information relating to optical characteristic values
inside the test object can be visualized. This makes it possible to
obtain an optical characteristic value distribution, particularly an
optical energy absorption density distribution inside the test object
with a high resolution.

[0004] Transducers utilizing piezoelectricity are typically employed as
detectors of acoustic waves. Transducers utilizing changes in capacity
are also being provided for general use.

[0005] In addition, a detector utilizing optical resonance has recently
been studied and reported (see Non-Patent Literature 1). This known
detector employs a technique in which acoustic waves are detected on the
principle of Fabry-Perot interferometer (hereafter, also referred to as
the FP method), and this detector is characterized by having broadband
reception performance, providing high-definition images.

[0006] However, the FP method has a drawback of requiring long time for
measurement. According to Non-Patent Literature 1, for example, in order
to acquire two-dimensional distribution data of photoacoustic waves, a
measurement light for evaluating optical reflectance is scanned by means
of a galvanometer. This means that, in order to acquire one piece of
volume data, optical resonance positions are raster scanned to acquire
data at the respective positions. At the same time, in order to set an
optimum wavelength at each of the measurement positions, the data are
acquired while changing the measurement wavelength for each of the
positions. It is reported that, according to this technique, it takes ten
minutes or more to obtain a three-dimensional image of a few millimeters
square.

[0007] In general, it is practically important for measurement equipment
to acquire data in as short period of time as possible. In particular,
when an object to be measured is a living body, the state of the test
object is successively changed by effects of body motion or the like.
Therefore, an adequate image cannot be obtained if it takes long time to
acquire data.

[0008] An attempt has been reported in which in order to collectively
acquire two-dimensional distribution of elastic waves, an acoustic
pressure of ultrasound waves acquired by a FP-type reception element is
detected by using a CCD camera as a two-dimensional array sensor (see
Non-Patent Literature 2).

[0011] As described in the section of Background Art, in an acoustic wave
detection apparatus using the FP method, it is very useful to use CCD or
the like to collectively acquire results of optical detection in a
two-dimensional plane in order to reduce the time required for
measurement. However, the inventors of this invention have found out, as
a result of our earnest studies conducted with a view to practical
application, that this technique has problems that are not described in
NPL 2.

[0012] According to the FP method, acoustic waves are received by a
reception film, and a slight variation in the reception film thickness
generated when an acoustic pressure of the waves reaches the film is
monitored optically to detect the acoustic pressure. This means that if
the reception film is formed into a thickness that is even slightly
different from a design value, the acoustic pressure cannot be measured
correctly. Since process variation usually occurs during formation of the
reception film, not a little variation exists in the film thickness even
on a single substrate. Nevertheless, as long as the variation in the film
thickness is within a design allowable value specified for a relevant
product, the reception film can be put in practical use.

[0013] However, the inventors of this invention have calculated the design
allowable value to reveal that in an acoustic wave detector using the FP
method, even existence of a film thickness distribution of as small as a
few nanometers affects the receiving sensitivity of the detector. This
means that, in order to collectively receive correct signals in a
two-dimensional plane, the film thickness distribution must be controlled
within a few nanometers. However, such precise control is very difficult
in actual film formation processes.

[0014] This invention has been made in view of the problems described
above, and it is an object of the invention to provide a technique
enabling acoustic wave detection capable of realizing a high sensitivity
even when a reception film has a film thickness distribution.

[0016] a resonator including a first mirror on which measurement light is
incident, a second mirror which is arranged to face the first mirror and
on which acoustic waves from an object are incident, an acoustic wave
reception layer interposed between the first mirror and the second
mirror, and a compensation layer; and

[0017] a detector for detecting a variation in an optical path length
between the first mirror and the second mirror that occurs in response to
deformation of the acoustic wave reception layer caused by incidence of
the acoustic waves,

[0018] wherein the variation in the optical path length due to a film
thickness distribution of the acoustic wave reception layer is
compensated by refraction in the compensation layer.

Advantageous Effects of Invention

[0019] According to this invention, a technique can be provided which
enables acoustic wave detection capable of realizing a high sensitivity
even if a reception film has a film thickness distribution.

[0020] Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a diagram showing an example of a configuration of a
conventional Fabry-Perot interferometer;

[0022] FIGS. 2A and 2B are diagrams showing an example of reflectance
properties of a Fabry-Perot interferometer;

[0023]FIG. 3 is a diagram showing an example of a configuration of a
Fabry-Perot interferometer according to this invention;

[0024]FIG. 4 is a diagram showing an example of a configuration of a
Fabry-Perot probe to which this invention is applicable;

[0025]FIG. 5 is a diagram showing an example of a configuration of a
living body information imaging apparatus to which this invention is
applicable;

[0026] FIGS. 6A to 6C are diagrams showing an example of a process of
fabricating an element according to an embodiment of this invention;

[0027] FIGS. 7A to 7E are diagrams showing an example of a control method
according to an embodiment of this invention;

[0028] FIG. 8 is a diagram showing an example of a time chart according to
an embodiment of this invention; and

[0029]FIG. 9 is a diagram showing an example of electrode formation
according to an embodiment of this invention.

DESCRIPTION OF EMBODIMENTS

[0030] <Basic Form of Configuring Reception Element>

[0031] Exemplary embodiments of this invention will be described with
reference to the drawings.

[0032] The term "measurement light" as used in this invention means light
that is used for measurement with a Fabry-Perot (FP) interferometer. The
measurement light includes all of incident light entering the FP
interferometer and reflected light reflected by the FP interferometer and
introduced into an array-type optical sensor.

[0033] Firstly, referring to FIG. 1, description will be made of a
conventionally known acoustic wave detecting element employing optical
resonance. A structure in which light is resonated between parallel
reflection plates as shown in FIG. 1 is called a FP interferometer.
Hereinbelow, an acoustic wave detector using this FP interferometer shall
be called a FP probe.

[0034] A polymer film 104 having a thickness d is interposed between a
first mirror 101 and a second mirror 102, whereby a resonator 103 is
formed. As shown in FIG. 1, the first mirror 101 and the second mirror
102 are arranged to face each other to define a cavity. Incident light
105 is emitted to the interferometer via the first mirror 101. A light
amount Ir of reflected light 106 can be represented by the following
formula (1).

[0035] In the formula (1), φ is represented by the following formula
(2):

[ Math . 2 ] Φ = 4 π λ 0 nd
( 2 ) ##EQU00002##

[0036] In the formulae (1) and (2), Ii denotes an incident light amount of
the incident light 105, R denotes a reflectance of the first mirror 101
and second mirror 102, λ0 denotes a wavelength of the incident
light 105 and reflected light 106, d denotes a distance between the
mirrors, and n denotes a refractive index of the polymer film 104. φ
corresponds to a phase difference when the light reciprocates between the
two mirrors.

[0037]FIG. 2A shows an example of a graph representing reflectance Ir/Ii
as a function of φ. As seen from the graph, the reflected light
amount Ir drops periodically and the reflectance becomes the minimum when
φ=2mπ (m is a natural number).

[0038] When acoustic waves 107 enter the FP probe, the inter-mirror
distance d is changed by deformation of the probe. This changes the value
of φ, which in turn changes the reflectance Ir/Ii. The incident
acoustic waves 107 can be detected by measuring the change in the
reflected light amount Ir by means of a photodiode or the like. As the
change in the reflected light amount is increased, the intensity of the
incident acoustic waves 107 becomes higher.

[0039] In order that the reflected light amount Ir is changed
significantly upon entrance of the acoustic waves 107, a rate of change
of reflectance Ir/Ii with respect to the change of φ must be high. In
FIG. 2, the rate of change becomes the highest, that is, the rate of
change exhibits a steep gradient at φm. Therefore, it can be
said that the element has the highest sensitivity at φm.

[0040]FIG. 2B shows a graph in which the reflectance Ir/Ii is represented
as a function of λ0. Matching the wavelength to λm
at which the rate of change of reflectance Ir/Ii becomes the highest
corresponds to matching the phase difference to φm, and the
sensitivity becomes the highest at λm.

[0041] Thus, in a FP probe, an optimum film thickness can be obtained once
a measurement wavelength to be used is determined. Referring to FIG. 2B,
for example, variation in film thickness must be controlled substantially
within ±0.05% when a light source with a single wavelength λA is
used. This value means that a considerably high accuracy is required for
the film formation technique.

[0042] When the reception surface of the FP probe is defined as the x-y
plane, a thickness at each position is represented by d(x,y), and
φm indicating an optimum sensitivity at each position is
represented by φm(x,y), the optimum sensitivity φm(x,y)
can be represented as the following formula (3).

[ Math . 3 ] Φ m ( x , y ) = 4
π λ A n d ( x , y ) ( 3 ) ##EQU00003##

[0043] Based on the description above, this invention is characterized in
that even if the film thickness d(x,y) varies from place to place,
φm assumes a constant value regardless of the place. Therefore,
the invention intends to cause the φm to assume a substantially
constant value in all the x-y coordinates by distributing the refractive
index n in the x-y plane. For this purpose, according to the invention,
an interferometer is operated while two layers of a reception layer and a
compensation layer are incorporated between resonators. Thus, the optical
path length in this case can be represented by a product of the
refractive index n and d that is a physical thickness. The refractive
index n is also distributed in the x-y plane. In other words, the optical
path length is made constant in the entire element by introducing the
concept as represented by the formula (4) below.

[Math. 4]

nd=n(x,y)d(x,y) (4)

[0044] Referring to FIG. 3, description will be made of a compensated FP
acoustic wave detecting element according to this invention. The basic
configuration is substantially the same as that shown in FIG. 1. An FP
interferometer 303 is interposed between two mirrors 301 and 302 arranged
to face each other. In the FP interferometer 303, a refractive index of a
layer 304 for receiving acoustic waves 307 is denoted by nr, a
refractive index of a compensation layer 309 is denoted by nc, a
refractive index of a support base 308 for the compensation layer is
denoted by ns, a film thickness of the reception layer 304 is
denoted by dr, a film thickness of the compensation layer is denoted
by dc, and a film thickness of the support base 308 for the
compensation layer is denoted by ds. In this case, a phase
difference can be represented by the formula (5) below.

[0046] In this case as well, the same properties are exhibited as those
shown in FIG. 2. When acoustic waves 307 enter the FP probe, the
inter-mirror distance d is changed. This changes the value of φ,
which in turn changes the reflectance Ir/Ii. The incident acoustic waves
107 can be detected by measuring the change in reflected light amount Ir
by means of a photodiode or the like. As the change in reflected light
amount is increased, the intensity of the incident acoustic waves 107
becomes higher.

[0047] When it is assumed that the wavelength λA of the
measurement light is a fixed value, the other parameters must be adjusted
in order to match the phase difference to φm. The refractive
indices nr and ns are material values determined according to
materials, while the thicknesses dr, dc, and ds are
parameters determined according to a manufacturing process. According to
the invention, therefore, the variation in optical path length due to
distribution of d caused by variation in the manufacturing process is
compensated by modulating the refractive index nc of the
compensation layer.

[0048] If the variation in the manufacturing process cannot be absorbed
enough only by the modulation of the refractive index nc of the
compensation layer, one more wavelength is added to the measurement light
and the region covered by the FP probe is divided so that the wavelength
λA is used in one subregion while the wavelength λB
is used in another subregion. This makes it possible to compensate a
wider area. The number of wavelengths may be increased further depending
on a degree of variation. Although the configuration is made complicated
by increasing the number of wavelengths, this measure should be employed
if it is more advantageous in terms of cost and output than using a laser
capable of continuously varying the wavelength. Materials to be used for
the compensation layer will be described later.

[0049] In the FP probe, a variation of reflected light amount is measured
only at a position irradiated with the incident light 105 (305) as
measurement light. Therefore, the spot irradiated with the incident light
defines a region having a receiving sensitivity. Accordingly,
two-dimensional distribution data of the acoustic waves can be obtained
by raster-scanning the incident light by means of a galvanometer or the
like. The two-dimensional distribution data of the acoustic waves thus
obtained is subjected to signal processing, whereby an image can be
obtained.

[0050] According to this invention, light of single wavelength can be used
as the incident light. Therefore, the entire surface of the element can
be simultaneously irradiated with the incident light and an image can be
acquired rapidly from the reflected light without the need of
raster-scanning by using a matrix type image sensor. Further, since a CCD
for digital cameras or a CMOS imaging element commonly used has a pixel
pitch of a few micrometers, a sufficiently high resolution can be
obtained.

[0051]FIG. 4 is a basic conceptual diagram illustrating a cross-sectional
configuration of the FP probe according to this embodiment of the
invention. A first mirror 401 and a second mirror 402 may be formed by a
dielectric multilayer film or a metal film. An acoustic wave reception
film 403 is present between the mirrors. The acoustic wave reception film
403 is preferably distorted significantly when elastic waves enter the FP
probe, and can be formed by an organic polymer film. The organic polymer
film can be formed of parylene, SU-8, polyethylene or the like. The
acoustic wave reception film 403 may be formed of an inorganic film as
long as it is deformed when receiving acoustic waves.

[0052] According to this invention, in addition to the conventional
configuration, an optical path length compensation layer 404 is provided
between the mirrors. This optical path length compensation layer is
arranged to compensate the film thickness distribution of the acoustic
wave reception film 403. A resonator is formed by the combination of the
layers present between the mirrors, including the acoustic wave reception
film 403, the optical path length compensation layer 404, and a layer 405
including the base for supporting the compensation layer.

[0053] The optical path length compensation layer 404 is preferably made
of liquid crystal from the viewpoint that it is easy to control from the
outside. For example, a nematic liquid crystal material widely used can
be used as follows. Firstly, a liquid crystal cell is fabricated by
combining, in an antiparallel fashion, two glass substrates which are
uniaxially parallel oriented by rubbing processing or the like. It is
known that when liquid crystal with positive dielectric anisotropy is
injected and a sufficiently high voltage is applied between the
substrates, the orientation of liquid crystal molecules is changed from a
substantially parallel direction to a substantially vertical direction to
the substrates. The refractive index in the uniaxially oriented direction
(extraordinary refractive index) indicates a refractive index
substantially in a major axis direction of the liquid crystal molecules
when no voltage is applied, whereas it indicates a refractive index
substantially in a minor axis direction of the liquid crystal molecules
when a sufficient voltage is applied so that the liquid crystal molecules
are oriented vertically to the substrates. This makes it possible to
continuously modulate the optical path length of this liquid crystal
element when polarized light is emitted in a direction in which the
uniaxial orientation is performed.

[0054] When the liquid crystal has a negative dielectric anisotropy, the
optical path length can be continuously changed by application of a
voltage in the same manner as described above by using substrates in
which the liquid crystal molecules are oriented substantially vertically
and which have pretilt angles in antiparallel directions.

[0055] Furthermore, the refractive index can be modulated regardless of
the direction of polarized light by using a publicly known
polymer-stabilized blue phase liquid crystal. Alternatively, it is also
possible to use a ferroelectric liquid crystal having a helical pitch
shorter than the wavelength.

[0056] Since in this invention any material can be used as long as it has
a function of modulating the refractive index, a material having no
liquid-crystallinity may be used. For example, the refractive index can
be modulated by using aqueous sucrose solution as the compensation layer,
and giving the compensation layer a sucrose concentration gradient in
accordance with the film thickness distribution in the acoustic wave
reception layer. Alternatively, a charged material exhibiting different
refractive indices depending on the concentration may be used in place of
sucrose, whereby the refractive index can be controlled by externally
giving a concentration gradient with use of electrophoresis.

[0057] In this case, electrodes and a driving device (not shown) are
provided in order to externally modulate the refractive index of the
compensation layer.

[0058] The FP probe as a whole is protected by a protection film 407. The
protection film 407 is formed by an organic polymer film of parylene or
the like or an inorganic film of SiO2 or the like that is formed
into a thin film. A substrate 406 on which the first mirror 401 is formed
may be made of glass or acryl. The substrate 406 is preferably formed
into a wedge shape in order to reduce the effect of optical interference
in the substrate 406. Further, the substrate 406 is preferably coated
with an antireflection coating 408 in order to avoid optical reflection
at the surface of the substrate 406.

[0059] <Basic Form of System Configuration>

[0060]FIG. 5 is a diagram for explaining a configuration example of an
imaging apparatus according to this embodiment.

[0061] The imaging apparatus according to this embodiment has an
excitation light source 504 which emits excitation light 503 to the test
object 501 to excite the photoacoustic waves 502. When the test object
501 is a living body, an optical absorber inside the test object 501 such
as a tumor or a blood vessel in the living body can be imaged. An optical
absorber on the surface of the test object 501 also can be imaged. The
optical absorber present inside or on the surface of the test object 501
absorbs part of the optical energy, whereby photoacoustic waves 502 are
generated. A FP probe 505 is provided for detecting these photoacoustic
waves 502. The FP probe is provided with a compensation layer for
compensating the aforementioned film thickness distribution, and can be
controlled from the outside. A controller 517 for controlling the FP
probe is also provided.

[0062] The FP probe 505 is enabled to detect acoustic pressure by applying
measurement light 506 thereto. A light source for measurement light 507
is provided for generating the measurement light 506. A controller 508 is
also provided for controlling the light source for measurement light. The
light source for measurement light 507 may be a single-wavelength light
source or a light source capable of switching wavelengths. The light
source for measurement light 507 further may be a light source capable of
continuously changing the wavelength. The switching of the wavelength and
turning on and off of light emission are performed by the controller 508.

[0063] Further, an array-type optical sensor 509 is provided for measuring
a light amount of the measurement light 506 emitted to and reflected by
the FP probe 505 and converting the measured light amount into an
electrical signal. An acoustic wave receiving apparatus is formed by
these components described above.

[0064] The acoustic wave receiving apparatus is further provided with a
signal processor 510 and an image display unit 511, whereby the imaging
apparatus is formed. This means that in the imaging apparatus according
to this embodiment, an electrical signal obtained by the array-type
optical sensor 509 is analyzed by the signal processor 510, and optical
characteristic value distribution information thus obtained is displayed
by the image display unit 511.

[0065] The measurement light 506 is enlarged by a lens 512, reflected by
the FP probe 505, and then is incident on the array-type optical sensor
509, whereby reflection intensity distribution on the FP probe 505 can be
obtained. An optical system is formed by a mirror 513, a half mirror 514
and so on. The optical system may be configured in any manner as long as
it is able to measure reflectance of the FP probe 505. For example, a
polarized light mirror and a wave plate may be employed in place of the
half mirror 514, or an optical fiber may be used. A position on the FP
probe 505 is associated with a pixel on the array-type optical sensor 509
by this optical system.

[0066] The array-type optical sensor 509 may be an optical sensor of
two-dimensional array type or one-dimensional array type. For example, a
CCD sensor or a CMOS sensor can be used as the array-type optical sensor
509. However, any other type of array-type optical sensors can be used as
long as it is able to measure a reflected light amount of the measurement
light 506 when the photoacoustic waves 502 are incident on the FP probe
505 and to convert the measured reflected light amount into an electrical
signal.

[0067] The distance between the mirrors of the FP probe 505 varies from
position to position. The refractive index is adjusted at each position
(at each associated pixel on the array-type optical sensor 509) by means
of the compensation layer so that the optical path length is fixed in the
surface of the element.

[0068] The excitation light 503 emitted to the test object 501 can be
light having such a wavelength that is absorbed by a specific component
among the components forming the test object 501. The excitation light
503 may be pulsed light. The duration of each pulse of the pulsed light
is in the order of from a few picoseconds to a few hundred nanoseconds.
When the test object is a living body, it is desirable to employ pulsed
light with a pulse duration of from several nanoseconds to several tens
of nanoseconds. A laser is preferred as the light source 504 generating
the excitation light 503, whereas a light emitting diode or a flash lamp
can be used in place of the laser.

[0069] Various lasers such as a solid laser, a gas laser, a dye laser, and
a semiconductor laser can be used as the laser for exciting photoacoustic
waves. It is made possible to determine a difference in optical
characteristic value distribution depending on wavelengths by using a dye
capable of converting oscillating wavelengths, an OPO (Optical Parametric
Oscillator) or a TiS (Titanium Sapphire).

[0070] The light source used for this purpose preferably has a wavelength
in a range of 700 nm to 1100 nm that is less absorbed by living body
tissues. When a target region for observation is a region close to the
surface of a living body or of a test object that is other than a living
body, the wavelength range can be set wider than the aforementioned
range, for example to a range of from 400 nm to 1600 nm. Further, an
ultraviolet range, a terahertz wavelength range, a microwavelength range,
and a radio wavelength range are also usable.

[0071] In FIG. 5, the excitation light 503 is emitted to the test object
from such a direction that the test object is not hidden behind the FP
probe 505. However, if the FP probe is made of a material that is
transmissive to the wavelength of the excitation light 503, the
excitation light 503 can be applied through the FP probe.

[0072] In order that photoacoustic waves 502 generated by the test object
501 are detected by the FP probe 505 efficiently, an acoustic coupling
medium is desirably provided between the test object 501 and the FP probe
505. Although in FIG. 5, water is used as the acoustic coupling medium
and the test object 501 is placed in a water bath 515, the invention is
not limited to this as long as an acoustic coupling medium is interposed
between the test object 501 and the FP probe 505. For example, a contact
gel for use in ultrasound diagnosis may be applied between the test
object 501 and the FP probe 505.

[0073] When the test object 501 is irradiated with the excitation light
503, photoacoustic waves (ultrasound waves) 502 are generated from the
inside of the test object by the test object absorbing a part of the
energy of the excitation light 503. The FP probe 505 detects these
photoacoustic waves 502 as a change in the reflected light amount of the
measurement light 506. The detected light amount is converted into an
electrical signal by the array-type optical sensor 509. Therefore,
electrical signal distribution in the array-type optical sensor 509
represents an intensity distribution of the photoacoustic waves 502
reaching the FP probe 505. In this manner, pressure distribution of the
photoacoustic waves 502 reaching the FP probe 505 can be obtained.

[0074] Further, the signal processor 510 calculates an optical
characteristic value distribution such as a distribution of positions or
sizes of the optical absorber in the test object 501, or a distribution
of optical absorption coefficients or optical energy accumulations, based
on the extracted electrical signal distribution in the array-type optical
sensor 509.

[0075] A universal back-projection algorithm, phasing and adding,
model-based image reconstruction or the like may be used as a
reconstruction algorithm for obtaining an optical characteristic value
distribution from the obtained electrical signal. It is also possible
that in view of the possibility that a region cannot be used as data,
where the film thickness exhibits severe abnormality, for example, due to
presence of a foreign matter in the acoustic wave reception film 403 of
the FP probe, imaging is performed after correcting the lost data part
during image reconstruction processing.

[0076] The signal processor 510 may be of any type as long as it is able
to store the distribution of time variation of an electrical signal
representing the intensity of the photoacoustic waves 502, and to convert
it into data of optical characteristic value distribution by means of
computing means.

[0077] Light with a plurality of wavelengths can be used as the excitation
light 503. In this case, an optical coefficient in the living body is
calculated for each of the wavelengths, and the values thus obtained are
compared with wavelength dependences intrinsic to the substances forming
the living body tissues, whereby the concentration distributions of the
substances forming the living body can be imaged. The substances forming
the living body tissues include glucose, collagen, and oxygenated and
deoxygenated hemoglobin.

[0078] It is desirable in this invention that the image display unit 511
is provided for displaying image information obtained by the signal
processing.

[0079] The use of the living body information imaging apparatus as
described above makes it possible to obtain a photoacoustic image in a
very short time with use of the FP probe 505.

[0080] <First Embodiment of Compensation Layer>

[0081] Subsequently, various embodiments of this invention will be
described, focusing on matters related to the compensation layer. A
method of using the compensation layer while activating the same will be
described.

[0082] FIG. 6 is a diagram showing an example of a process for fabricating
elements used in this embodiment. The elements can be obtained by forming
films in the order as described below. A transparent electrode used
herein is covered with a film over its entire effective reception area.
FIGS. 6A and 6B illustrate an element 1 and an element 2, respectively,
and FIG. 6c illustrates a cell formed by the combination thereof.

[0083] <<Fabrication of Elements>>

[0084] (First Substrate)

[0085] 1: A glass substrates (601) with a transparent electrode (603) is
prepared and a parylene film (602) is formed on the surface opposite to
the transparent electrode.

[0086] 2: A dielectric multilayer film mirror (604) is formed on the
parylene film (602).

[0087] 3: A protection film (606) is formed on the dielectric multilayer
film mirror (604).

[0088] 4: A horizontally-oriented film (605) is formed on the transparent
electrode (603) and orientation processing (607) is carried out.

[0089] (Second Substrate)

[0090] 1: A dielectric multilayer film mirror (611) is formed on a glass
substrates (610).

[0091] 2: A transparent electrode (612) is formed on the dielectric
multilayer film mirror (611).

[0092] 3: A horizontally-oriented film (613) is formed on the transparent
electrode (612), and orientation processing (614) is carried out.

[0093] (Cell Assembly) 1: Spacer beads (617) with a diameter of a few
micrometers are sprayed onto the first substrate (609).

[0094] 2: A sealing material (616) is applied on the periphery of the
second substrate (615).

[0095] 3: The first substrate (609) and the second substrate (615) are
combined and bonded to each other with their orientation processing
directions being aligned antiparallel.

[0096] 4: Heat treatment is performed to thermally cure the sealing
material (616).

[0097] 5: A nematic liquid crystal material (618) with positive dielectric
anisotropy is injected through a liquid crystal inlet (not shown) and
then the inlet is sealed off.

[0098] 6: Electrode wires (619) are extended out of the transparent
electrodes (603, 612) of the upper and lower substrates and connected to
an AC voltage supply (620).

[0099] In this element, the optical path length of the liquid crystal
layer can be varied by voltage modulation. Accordingly, when reflectance
properties are measured with use of this element and voltage is plotted
along the abscissa while reflectance is plotted along the ordinate, a
similar profile to that shown in FIG. 2 can be obtained. This enables
nc(x,y) in the formula (5) to be modulated by voltage. In liquid
crystal materials for use in common displays, the minimum value of
nc is about 1.5 corresponding to a refractive index n∥ in a
uniaxial direction of liquid crystal molecules, while the maximum value
is about 1.6 corresponding to a refractive index n'' in a major axis
direction of liquid crystal molecules. Materials with a refractive index
anisotropy Δn of about 0.1 are widely used. Some materials having a
refractive index anisotropy Δn of 0.3 or more have been developed.

[0100] According to this invention, any liquid crystal material can be
used since the film thickness can be adjusted to obtain optimum
conditions.

[0101] <<Adjustment of Compensation Amount>>

[0102] A FP element (621) that is a cell obtained by the aforementioned
process has a distribution in optical path length during optical
interference in the surface of the element, owing to the film thickness
distribution of the parylene film (602) and the cell thickness
distribution of the liquid crystal layer (618) itself functioning as a
compensation layer. In order to compensate this distribution, an amount
of voltage applied to the liquid crystal is adjusted.

[0103] In this element, values are variable according to a polarization
axis of the measurement light due to effects of refractive index
anisotropy of the liquid crystal. Therefore, the direction of the
polarization axis of the measurement light is preliminarily matched with
the orientation processing direction (the direction of the extraordinary
refractive index) of the liquid crystal with use of a polarizing plate.
This makes it possible to match the major axis direction of the liquid
crystal molecules with the polarization axis, and hence the optical path
length can be varied by applying a voltage to the liquid crystal layer.

[0104] A distribution amount is preferably measured for each pixel of the
array-type optical sensor (509), but it may be measured for a plurality
of pixels each time.

[0105] Measurement light with a predetermined wavelength is emitted to the
FP probe (505) and reflected light therefrom is measured with the
array-type optical sensor (509). An amount of light incident into the
sensor is measured with the applied AC voltage being changed, and a
voltage-reflectance profile is measured. In this manner, a voltage value
which gives φm can be obtained.

[0106] A look-up table (LUT) is produced by performing this measurement
for all the pixels and stored in a storage medium. When the used liquid
crystal has properties prone to be changed according to temperature, a
similar LUT is produced by varying the temperature.

[0107] <<Usage of Compensated FP Probe>>

[0108] In the LUT described above, pixels having the same optimum voltage
value are grouped together, and acoustic wave data is acquired for each
group. In this case, acoustic waves can be measured even if the voltage
is not completely the same but slightly different from the voltage that
gives φm. Therefore, when a required accuracy for the apparatus
is not high, voltage values included in a trough of the voltage-light
amount curve can be deemed to belong to the same group when performing
the measurement.

[0109] This means that although measurement is substantially always
possible under the condition of φm and a desirable sensitivity
is always ensured when the voltage value to be set is controlled
minutely, the measurement time is increased since the number of divided
voltages is increased. In contrast, when the voltage value is controlled
more roughly, variation in sensitivity is increased but the measurement
time is reduced. Thus, it is desirable to consider such trade-off when
designing a device so as to achieve its optimum conditions.

[0110] A measurement sequence will be described with reference to FIG. 7.
FIG. 7A is a top view of the FP probe. The circle dotted lines in FIG. 7A
indicate contour lines relating to the optical path lengths in the FP
probe. The FP probe has a film thickness distribution in which optical
path length is longer, that is, the film thickness is larger in its
central region, while the optical path length is shorter, that is, the
film thickness is smaller in its peripheral region. FIGS. 7B to 7E show
cross sections of these in a simplified manner. Voltage application
groups are established according to these contour lines.

[0111] A first photoacoustic wave signal exciting laser is emitted and
photoacoustic waves thus obtained are imaged with pixels belonging to a
first group. Specifically a region indicated by an encircled number 1 in
FIGS. 7A and 7B is imaged. Hereafter, the encircled number 1 shall be
represented as (1). Encircled numbers 2 to 4 shall also be represented as
(2) to (4), respectively. While the regions other than (1) exhibit the
same orientation state of the liquid crystal layer as the orientation
state in the region (1), these region can be ignored since they are not
used in image processing. Specifically, an optical intensity is read from
the top face of the element with the array-type optical sensor, and only
information of pixels corresponding to the region (1) is used in image
processing, whereas information of the regions (2) to (4) is not used.

[0112] Subsequently, second photoacoustic wave signal exciting laser is
emitted and photoacoustic waves thus obtained are imaged with pixels
corresponding to the second group. Specifically, the region indicated by
(2) in FIGS. 7A and 7C are imaged. While the regions other than (2)
exhibit the same orientation state of the liquid crystal layer as the
orientation state in the region (2), these region can be ignored since
they are not used in image processing. Specifically, an optical intensity
is read from the top face of the element with the array-type optical
sensor, and only information of pixels corresponding to the region (2) is
used in image processing, whereas information of the region (1), (3), and
(4) is not used.

[0113] Likewise, the regions (3) and (4) are imaged, whereby it is made
possible to seta substantially optimum optical path length for each of
the regions (1) to (4) and to receive acoustic waves. In this manner,
reception of the acoustic waves is performed a plurality of times
corresponding to the orientations of the liquid crystal layer for the
respective regions, in other words, corresponding to the compensation
amounts of the optical path length for the respective regions. The
results thus obtained are put together in the course of data analysis so
that a signal of the entire surface of the element is obtained.

[0114] Although the FP probe is divided into four regions in this example,
it may be divided into an arbitrary number of regions (N regions) and
data can be obtained in the same manner. When a pulse repetition
frequency of the photoacoustic wave signal exciting laser is represented
as f(Hz), each data can be obtained at a frequency of f/N(Hz).

[0115] If the array-type optical sensor is capable of acquiring images
rapidly enough so that data can be obtained following acoustic wave
oscillations, images can be acquired sequentially at a frame frequency of
f/N(Hz).

[0116] In contrast, if the image input of the array-type optical sensor is
too slow to follow acoustic wave oscillations, pulsed light may be used
as the measurement light to acquire data utilizing the principle of
stroboscopic imaging. FIG. 8 is a timing chart of stroboscopic imaging.
The photoacoustic wave signal exciting laser is emitted at N(Hz). When it
is assumed that the test object will not move during the imaging, the
photoacoustic wave signal is output repeatedly at the same intensity and
with the same phase.

[0117] Therefore, the acoustic pressure can be measured at different
timings by using an output of the photoacoustic wave signal exciting
laser as a trigger signal, and acquiring data by emitting pulsed laser as
measurement light to the FP probe while gradually delaying the laser
emission timing.

[0118] The uppermost part of FIG. 8 shows outputs of the photoacoustic
excitation laser. The photoacoustic signal from the absorber irradiated
with the photoacoustic excitation laser reaches the probe with a delay of
a predetermined time. This is illustrated with waveforms in the second
part from the top of FIG. 8.

[0119] The third part from the top of FIG. 8 shows waveforms of the pulsed
laser for measurement light. Since light is directed onto the FP probe
only at these moments, reflected light therefrom reaches the array-type
optical sensor. The optical intensity of the light reaching the
array-type optical sensor is shown in the lowermost part of FIG. 8. It is
assumed here for the purpose of simplicity that the intensity of
reflected light is proportional to the photoacoustic wave signal.
Two-dimensional distribution of the reflected light thus reflected is
accumulated in an image memory.

[0120] In the next measurement, the timing to emit the pulsed laser for
measurement is delayed slightly from the trigger in comparison with the
previous measurement, and the intensity of the reflected light is
measured. Since the photoacoustic wave signal is generated repeatedly in
the same waveform, two-dimensional distributions of reflected light can
be obtained at different timings by delaying the timing to emit the
pulsed laser for measurement light.

[0121] The process of emitting the measurement light while delaying the
timing is further repeated to accumulate data in the memory, whereby
photoacoustic wave signals corresponding to one cycle can be obtained. By
organizing these signals, time change of the intensity of reflected light
from the FP probe can be obtained for each pixel. When the number of
divisions for observing photoacoustic wave signals with a stroboscope is
indicated by D, the data is acquired at f/(N×D) (Hz).

[0122] When the signal is averaged m times for each acquisition point, the
data is acquired at f/(N×D×m) (Hz). Therefore, the use of the
photoacoustic wave signal exciting laser capable of rapid repeated
emissions makes it possible to acquire data at a practical speed.

[0123] <Second Embodiment of Compensation Layer>

[0124] <<Fabrication of Elements>>

[0125] In the second embodiment, glass substrates with stripe-patterned
transparent electrodes are used as the first and second substrates.
Except this point, the same process as that of the first embodiment is
used to fabricate a cell. When assembling a cell, the two substrates are
bonded together such that their stripe electrodes are orthogonal to each
other to form a simple matrix formation. As shown in FIG. 9, the
electrodes of the first substrate are used as common (COM) electrodes
901, and the electrodes of the second substrate are used as segment (SEG)
electrodes 902.

[0126] In this embodiment, driving can be performed by using a simple
matrix liquid crystal driver that is typically used for
super-twisted-nematic (STN) liquid crystal or the like. A COM driver is
mounted on the first substrate and a SEG driver is mounted on the second
substrate.

[0127] When a region where matrix electrodes intersect is defined as a
pixel of liquid crystal, each liquid crystal pixel preferably corresponds
to a pixel of the 2D imaging element in one-to-one relationship. However,
since the liquid crystal layer here is provided for the purpose of
compensating the film thickness distribution, the liquid crystal pixels
may be arranged more coarsely than the pixels of the imaging element if
the film thickness distribution varies gradually.

[0128] <<Adjustment of Compensation Amount>>

[0129] Voltage-reflectance profile is measured in the same manner as in
the first embodiment. A look-up table (LUT) is produced by using this
measurement result, and stored in a storage medium.

[0130] When the used liquid crystal has properties prone to be changed
according to temperature, a similar LUT is produced by varying the
temperature.

[0131] At the same time, driving conditions for performing simple matrix
driving are obtained. A method for applying a voltage to each pixel may
be a typical driving method for simple matrix liquid crystal used for
common liquid crystal displays. In this case, it is known that a voltage
on-off ratio is determined according to the formula (7) below.

[ Math . 7 ] V ON / V OFF = N + 1 N
- 1 ( 7 ) ##EQU00006##

[0132] If the number of the lines in the COM is increased, it becomes
impossible to ensure a sufficient on-off ratio, that is, a sufficient
difference in optical path length, to compensate the parylene film
thickness.

[0133] Therefore, when fabricating the element, the thickness of the
liquid crystal cell is set to a value required for ensuring an optimum
compensation amount in view of the number of COM lines and the parylene
film thickness distribution. Specifically, in the case of the
horizontally oriented liquid crystal of this embodiment, a maximum value
of the optical path length is represented by n(VOFF)dc, and a
minimum value is represented by n (VON)dc. Thus, a
compensatable range of optical path length is represented by
{n(VON)-n(VOFF)}dc, wherein n (VOFF) denotes an
average extraordinary refractive index of the liquid crystal layer in the
off state during matrix driving, and is a component contributing to the
optical path length when the polarized light direction of the measurement
light is matched with the orientation processing direction. Likewise,
n(VON) denotes an extraordinary refractive index in the on state.
Thus, the compensatable range can be ensured by setting d to a great
value. However, if d is too great, a problem of deterioration of the
response speed will arise. Therefore, optimum conditions should
preferably be employed.

[0134] As described above, the number of scanning lines and the on-off
ratio are in trade-off relationship. Therefore, if a sufficient on-off
ratio cannot be ensured on the entire surface of the FP probe region, it
is useful to increase the driving duty ratio by reducing the number of
effective COM lines. For example, it may be useful to simultaneously
drive those lines in which the film thickness is substantially constant.
This means that for a region that is determined to have little film
thickness distribution by a preliminarily measurement, predetermined
groups of COM lines can be selected at the same time and compensated with
the same voltage. This makes it possible to increase the on-off ratio in
order to reduce the number of duties involved in the driving.

[0135] Alternatively, the entire element may be divided into N blocks, so
that a whole image is formed by using N fields to form a single image.
This means that, the COM lines are divided into N blocks and driven when
acquiring an image. In each field, COM lines of a number corresponding to
one N-th of the total number of the COM lines are driven. A light amount
is measured for each field by means of an image sensor and thus an image
is acquired N times at separate locations, whereby a compensation amount
is determined.

[0136] If the compensation amount is still deficient, the number of
wavelengths of the measurement light may be increased by one or more in
the same manner described above.

[0137] The driving method and the compensation amount are determined as
described above and recorded as a LUT in a storage medium.

[0138] <<Usage of Compensated FP Probe>>

[0139] Acoustic waves can be detected after driving the compensation layer
with the aforementioned simple matrix driving method and keeping the
optical path length uniform in the FP probe surface. When divided into N
blocks, simple matrix driving is performed for each of the divided areas,
and an acoustic wave signal of each area is received and stored in a
memory. The simple matrix driving is performed for the other blocks to
receive acoustic wave signals and an image of the entire of one element
is formed by using the data of the N fields.

[0140] Like the first embodiment, stroboscopic observation is possible
when the image acquisition of the array-type optical sensor is slow.

[0141] <Third Embodiment of Compensation Layer>

[0142] <<Fabrication of Elements>>

[0143] According to this third embodiment, an active matrix substrate for
liquid crystal display having a thin-film transistor (TFT) element
arranged thereon is used as the first substrate, while a substrate having
a transparent electrode formed on the entire surface thereof is used as
the second substrate. Except this point, the same process as that of the
first embodiment is used to fabricate a cell. Gate electrodes are formed
in row direction and source electrodes are formed in column direction of
the first substrate.

[0144] A liquid crystal layer according to this embodiment has the same
element configuration as that of a common active matrix driving liquid
crystal element. The first substrate having a TFT element arranged
thereon is provided a gate driver that is mounted in row direction and
with a source driver that is mounted in column direction, in order to
apply a voltage in a thickness direction of the cell between two
substrates having patterned transparent electrodes thereon in the same
manner as when driving the twisted nematic (TN) liquid crystal. The
second substrate is kept at a potential corresponding to the optimum
condition in the TFT driving.

[0145] The orientation processing direction and the liquid crystal to be
used are the same as those of the first and second embodiments described
above.

[0146] Although it is desirable that pixels of the liquid crystal layer
correspond to pixels of the array-type optical sensor in one-to-one
relationship, the liquid crystal pixels may be arranged more coarsely
than the pixels of the imaging element if the film thickness distribution
varies gradually since the liquid crystal layer is for compensating the
film thickness distribution.

[0147] <<Adjustment of Compensation Amount>>

[0148] An optimum amount of voltage to apply is obtained in the same
manner as in the first embodiment and stored in a storage medium as a LUT
for each liquid crystal pixel.

[0149] <<Usage of Compensated FP Probe>>

[0150] Acoustic waves can be detected after the cell used in this
embodiment is active-matrix driven and the optical path length is kept
uniform in the FP probe surface. Like the first embodiment, stroboscopic
observation is possible when the image acquisition of the array-type
optical sensor is slow.

[0151] <Fourth Embodiment of Compensation Layer>

[0152] The second and third embodiments described above relate to a
configuration in which one type of compensation voltage is applied to
each pixel of the liquid crystal layer. However, if each liquid crystal
pixel is large in size, that is, each pixel is so coarse relative to the
variation in thickness distribution of the parylene film that the optimum
compensation amount varies within the pixel, each liquid crystal pixel
can be divided into a plurality of fields to obtain data from each of the
fields. According to this, the entire element is space-divided with use
of the cell configuration according to the second or third embodiment,
and each pixel is time-divided by introducing the concept of the first
embodiment to acquire data from each region, whereby more delicate
compensation is enabled.

[0153] It should be noted that this technique requires that the pixel
pitch of the array-type optical sensor is smaller than that of the liquid
crystal pixel.

[0154] <Fifth Embodiment of Compensation Layer>

[0155] In the foregoing embodiments, generally-used nematic liquid crystal
for liquid crystal display can be employed, and this liquid crystal is
used in practice while being applied with a voltage. This fifth
embodiment described below relates to a method in which necessary
conditions for the compensation layer are incorporated in the fabrication
process, and this state is fixed after use.

[0156] <<Fabrication of Elements>>

[0157] A matrix electrode as described in the second or third embodiment
is used. A liquid crystal material used here is a liquid crystal material
having a phase series consisting of isotropic phase, nematic phase, and
smectic A phase from the high-temperature side.

[0158] After fabrication of an element, the element is heated to transform
the liquid crystal into the nematic phase, and driving is performed under
optimum conditions which can compensate the thickness of the parylene
film so that the phase is changed to the smectic A phase while applying a
drive voltage. When a smectic layer structure appears during the phase
change, the direction of liquid crystal molecules in the nematic phase
and the direction of molecules in the smectic A phase may be slightly
misaligned with respect to the inclination angle of the liquid crystal
molecules from the substrate. When this occurs, batonets (substantially
elliptical sea-island structures formed in a layer normal direction along
with crystal growth in a smectic layer when during first order phase
transition from the nematic phase to the smectic phase) may grow to form
a layer. Therefore, the voltage application conditions should be
determined in view of the properties of the used material.

[0159] <<Usage of Compensated FP Probe>>

[0160] When the liquid crystal is gradually cooled while applying voltage
so that the phase is changed to the smectic A phase, the orientation is
stabilized by the layer structure and hence this stabilized state is kept
even after the voltage is turned off. This enables the element to be used
as a FP probe without the need of applying compensation voltage.

[0161] Although the smectic liquid crystal phase is used in this
embodiment, any other liquid crystal layer phase or solid phase may be
used as long as its orientation state can be fixed after determination of
the orientation. For example, liquid crystal materials such as discotic
liquid crystal, side-chain polymer liquid crystal, and main-chain polymer
liquid crystal can be used. While these may be used in a liquid crystal
phase, it is desirable, when used in a solid phase, to use a material in
which the phase is changed to the solid phase by vitrification transition
instead of crystallization transition so that the orientation state in
the liquid crystal phase is maintained even after the phase is changed to
the solid phase.

[0162] As is described above, even using a material having no liquid
crystallinity, the refractive index distribution can be imparted and
compensated. For example, the refractive index distribution can be
imparted and compensated by imparting a concentration gradient of an
organic substance such as sucrose according to the film thickness
distribution of the acoustic wave reception layer, or by imparting a
concentration gradient from the outside by means of electrophoresis while
using a charged material having a refractive index which varies according
to a concentration. When using such a material in a liquid state, the
concentration distribution may possibly be lost due to convection or
diffusion. Therefore, it is desirable to use the material after taking
necessary measures to preserve the state of refractive index distribution
by providing barriers at predetermined intervals so as to prevent
diffusion or by solidifying with agar or the like as soon as the
concentration distribution is imparted.

[0163] While five exemplary embodiments of the invention have been
described, the invention is not limited to these embodiments but various
other materials can be used. For example, in the first to fourth
embodiments, parallel-oriented ECB (Electrically Controlled
Birefringence) liquid crystal is used when liquid crystal is to be used.
However, various other liquid crystal modes such as VA (Vertical
Alignment) mode, bend-oriented mode, and HAN (Hybrid Aligned Nematic)
mode can be used.

[0164] The adjustment of compensation amount described above can be
affected by variation with time. Therefore, periodic review of the LUT is
desirable not only before the factory shipment but also during usage.

[0165] Although in the embodiments, a layered structure of an acoustic
signal reception layer and a compensation layer is employed, it is also
possible to assemble a mirror optical system and an acoustic signal
reception unit and a compensation unit are formed by separate elements.

[0166] It is made possible to acquire a high-resolution photoacoustic
image very rapidly by using a living body information imaging apparatus
of such a configuration.

[0167] When the apparatus is used for medical application, a water bath as
shown in FIG. 5 is not used. Instead, an acoustic impedance-matching gel
is applied on the test object, that is, an affected part of the body, and
the FP probe 505 is placed in contact therewith to perform imaging. For
this purpose, not only the matching gel but also any other materials can
be used as long as they are able to provide acoustic matching between the
affected part and the FP probe 505.

[0168] Further, although the embodiments have been described focusing on
reception of a photoacoustic wave signal, any other signals are
detectable as long as they are elastic waves. Therefore, the invention is
also applicable to medical probes for ultrasound echography and
ultrasound probes for non-destructive inspection. Further, since this
element is a broadband element, it is applicable to microphones or
stethoscopes for detecting oscillation of audible acoustic waves.

Example 1

[0169] This example 1 is provided by the configuration described in
relation to the first Embodiment.

[0170] In the example 1, according to the invention, a sample to be imaged
is prepared as a test object, in which 1% aqueous solution of Intralipid
is solidified with agar and a light-absorbing rubber wire with a diameter
of 300 μm is placed therein. The sample is placed within water.

[0171] A dielectric multilayer film is used as the first and second
mirrors of the FP probe. This dielectric multilayer film is designed to
have a reflectance of 95% or more in the range of 900 to 1200 nm. BK7 is
used as the substrate of the FP probe, and antireflection coating is
applied on the opposite surface of the substrate from the surface on
which the dielectric multilayer film is formed such that the reflectance
is 1% or less in the range of 900 to 1200 nm. Parylene C is used as a
spacer film between the mirrors and the spacer film has a thickness of 30
μm. Parylene C is also used as a protection film of the probe.

[0172] MLC-6608 (manufactured by Merck) is used the liquid crystal
material for the compensation layer. Since this liquid crystal material
has negative dielectric anisotropy, a vertically oriented film is used
and orientation processing is performed so that the two substrates are
antiparallel with each other. A cell is thus assembled and the thickness
of the cell is 10 micrometers. The AC voltage supply can be modulated in
the range from 0 V to 10 V.

[0173] A laser diode capable of continuously oscillating at a wavelength
of 915 nm is used as the light source for measurement light which emits
measurement light for measuring a reflected light amount of the FP probe.

[0174] A high-speed CCD camera is used as the array-type optical sensor,
which has 100×100 pixels.

[0175] Measurement light is emitted and a light amount detected by the CCD
is monitored while changing the voltage as required. Voltage-reflectance
characteristics are recorded and a voltage value at which an optimum
state is realized is found. Thus, a LUT is produced for each CCD pixel.

[0176] After that, the test object is irradiated with excitation light,
and measurement of photoacoustic waves is started. The excitation light
source of the light emitted to the test object is a titanium-sapphire
laser. The emitted pulsed light has a repetition frequency of 10 Hz, a
pulse width of 10 ns, and a wavelength of 797 nm.

[0177] A thickness distribution of about 100 nm occurs in the parylene
film fabricated in this example. Therefore, data acquisition is performed
by dividing the region into 10 blocks on the element.

[0178] Using the distribution of the photoacoustic wave signal obtained by
the measurement, image reconstruction is performed by means of universal
back-projection algorithm. The reconstruction is performed with the voxel
pitch set to 0.5 mm. In this manner, the rubber wire in the agar
containing 1% Intralipid as a light diffusion medium can be imaged in the
imaging area with a diameter of 2 cm.

[0179] In this example 1, driving under optimum conditions is possible
according to the techniques described in the foregoing embodiments, and
thus acoustic signal data can be obtained with a desirable sensitivity.

[0180] The time required for imaging in the example 1 is less than one
minute, and it can be seen that it is faster than conventionally known
raster-scan systems.

Example 2

[0181] This example 2 is provided by the configuration of the FP probe
described in relation to the second embodiment.

[0182] The apparatus configuration and the test object used in this
example are the same as those described in example 1. The liquid crystal
layer is divided into 100×100 pixels and simple-matrix driven.

[0183] A thickness distribution of about 100 nm occurs in the parylene
film fabricated in this example. Therefore, data acquisition is performed
by dividing the region into 10 blocks on the element.

[0184] Using the distribution of the photoacoustic wave signal obtained by
the measurement, image reconstruction is performed by means of universal
back-projection algorithm. The reconstruction is performed with the voxel
pitch set to 0.5 mm. In this manner, the rubber wire in the agar
containing 1% Intralipid as a light diffusion medium can be imaged in the
imaging area with a diameter of 2 cm.

[0185] In this example 2, driving under optimum conditions is possible
according to the techniques described in the foregoing embodiments, and
thus acoustic signal data can be obtained with a desirable sensitivity.

[0186] The time required for imaging in this example 2 is less than thirty
seconds, and it can be seen this is faster than conventionally known
raster-scan systems.

Example 3

[0187] This example 3 is provided by the configuration of the FP probe
described in relation to the third embodiment.

[0188] The apparatus configuration and the test object used in this
example are the same as those described in the example 1. The liquid
crystal layer is divided into 100×100 pixels and simple-matrix
driven.

[0189] A thickness distribution of about 100 nm occurs in the parylene
film fabricated in this example. Therefore, data acquisition is performed
by dividing the region into 10 blocks on the element.

[0190] Using the distribution of the photoacoustic wave signal obtained by
the measurement, image reconstruction is performed by means of universal
back-projection algorithm. The reconstruction is performed with the voxel
pitch set to 0.5 mm. In this manner, the rubber wire in the agar
containing 1% Intralipid as a light diffusion medium can be imaged in the
imaging area with a diameter of 2 cm.

[0191] In this example 3, driving under optimum conditions is possible
according to the techniques described in the foregoing embodiments, and
thus acoustic signal data can be obtained with a desirable sensitivity.

[0192] The time required for imaging in this example is less than twenty
seconds, and it can be seen that it is faster than conventionally known
raster-scan systems.

[0193] As described in the foregoing examples, according to the
configuration of the invention, the optical length required for resonance
can be maintained substantially constant in a two-dimensional plane even
if a film thickness distribution exists due to variation in process of
formation of the reception film or the like. This makes it possible to
measure the reflectance gradient under a steep condition, and hence a
high sensitivity characteristic can be realized.

[0194] If the optical path length is uniform, only one wavelength is
required for the measurement light. Even if a plurality of measurement
wavelengths are used due to deficiency in correction amount, the number
of wavelengths can be significantly reduced in comparison with the case
in which no compensation layer is used. This contributes to cost
reduction of the apparatus. If the cost is the same, this contributes to
improvement of sensitivity since a higher output light source can be
employed.

[0195] Further, this invention is able to provide a stable apparatus since
the apparatus is not only capable of compensating variation in film
formation but also capable of absorbing various variable factors such as
variation in characteristics due to change of ambient temperature,
variation in characteristics due to variation with time of the element,
assembly error when the element is incorporated in the apparatus.

[0196] The foregoing description has been made with a focus on a
configuration example of the living body information imaging apparatus
used for a living body as a test object. This enables imaging of optical
characteristic value distribution in the living body and concentration
distribution of substances making up living body tissues for the purpose
of diagnosis of tumor or blood vessel disease or follow-up of chemical
treatment, and thus the apparatus according to the invention is usable as
medical diagnostic imaging equipment.

[0197] Further, it will be easy for those skilled in the art to apply the
invention to non-destructive inspection or the like for inspecting a
non-living substance as a test object.

[0198] As described above, the invention is widely applicable as an
inspection device.

[0199] While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0200] This application claims the benefit of Japanese Patent Application
No. 2011-117942, filed on May 26, 2011, which is hereby incorporated by
reference herein in its entirety.